Historical Articles

November, 1954 issue of Plating

 

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THE TECHNOLOGY OF LIQUID BUFFING COMPOSITIONS*

Robert V. Twyning, Twyning Chemical Specialties Company, Belvidere, Illinois.

*Presented at the Forty-First Annual Convention of the American Electroplaters’ Society, July 14, 1954

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The technology of liquid buffing compositions is a study in colloids, which subject has entertained the minds of some of the greatest physicists and chemists such as Kruyt, Helmholtz, Einstein, Debye, Langmuir, Freundlich, Zsigmondy, and Guoy over the past eighty years. It is advisable in the study of colloids that we revert to the academics of this branch of chemistry and differentiate between the two types of particles, suspensoids and emulsoids. Both emulsoids and suspensoids are sols-and differ from each other tremendously in their physical behavior. The viscosity of a suspensoid solution varies only slightly from water itself, i.e., 5 per cent, while the viscosity of an emulsoid varies tremendously, so much so that the addition of as little as 1/2 per cent of natural or synthetic gums will raise the viscosity sufficiently to become immeasurable. The addition of minute amounts of electrolytes to a suspensoid causes precipitation, while emulsoids are affected only slightly but will coalesce upon the addition of sufficient quantity of electrolyte, especially that containing the di- or trivalent cations. Suspensoids which have precipitated are in many cases not redispersable and in other cases only with great difficulty, while emulsoids which have coalesced can be regenerated easily. The stability of a suspensoid is dependent strictly on its electric charge (i.e., silica and gold sols being negative) and having lost its given charge precipitation occurs, while emulsoids have either a positive or negative electric charge and in some cases none at all (positive charge in acid solutions, negative charge in alkaline solutions). The effect which causes emulsoids to be inherently stable becomes evident when we consider that starches and gums undergo swelling in the presence of water. Hydration of such materials is a natural function, and the forces involved by their nature tend to keep the particles from coalescing. -Suspensoids are continuously in vigorous motion while emulsoids show no sign of motion when sighted through an ultra microscope.

The Greek terms used to denote the physical actions of the elements within a molecule having properties bearing on emulsification are hydrophobic (meaning water hating), hydrophilic (water loving), liophobic and lypophobic (solvent hating), and liophilic and lypophilic (solvent loving). When the term liophobic colloid is used, reference is made to a suspensoidal particle wherein water is the dispersing medium but the suspended particles such as metals and/or their oxides have little or no affinity toward water. In the original Greek lyo meant ”to loosen,” and it becomes clear that liophilic when used as a synonym for emulsoid refers to starches and gums which hydrate readily and ”love to loosen” and become disaggregated. As stated above, the elements within a molecule which are termed hydrophilic are normally one of the following polar groups, namely, hydroxyl, amine, carbonyl, mercapto, carboxyl, and functional groups of condensed ethylene oxide; hydrophobic elements are the nonpolar paraffinic or olefinic chains, in some cases containing even condensed propylene oxide within the hydrophobic element.¹

The function of an emulsifier is twofold, namely, to allow for ready preparation of the emulsion and also to control the type that is formed, whether oil-in-water or water-in-oil. The emulsifier reduces the interfacial tension of the two immiscible phases, thereby making the emulsion easy to prepare. Inter-facial tension is the numerical difference between the surface tension of two liquids to he emulsified. Water at 73 dynes per square centimeter and oil at 45 dynes per square centimeter have an inter-facial tension of 28 dynes. The function of the emulsifier is to reduce this 28 dynes to one or two dynes per square centimeter. If inter-facial tension measurements are made after the emulsifier has reduced the value to one or two dynes per square centimeter then it is found that the error introduced by the addition of the third phase (platinum ring) is in such magnitude as to give unreliable results.

All genuine rigorous emulsifiers are micro heterogeneous and are colloidally dispersible. They must be heterogeneous in order not to be highly soluble in either the dispersed or continuous phase. Their elements must be of such composition as to cause the molecule to be partially, but not totally, soluble in each phase. When this requirement is met the molecules are then vigorous and accumulate at the inter-facial boundary of the dispersed phase. Fatty soaps are of this nature and have been our most prominent emulsifiers. It is interesting to note here that charcoal and soot have been used-as emulsifiers just as effectively as fatty soaps; however, it is imperative to add that carbon in its various allotropic forms yields a positively charged suspensoid and that poor stability results if carbon is co-dispersed with finely divided siliceous abrasives which have an inherent negative charge. Advantage can be taken of the existence of this condition when, in the formulation of a solid or bar type buffing composition, it is found necessary to neutralize the extremely high electrostatic charge of the fine ends of tripoli powder (tripoli powder has the greatest dipole moment of all finely divided siliceous materials) which attach themselves electrostatically to the surface of buffed metal after the cleaning operation, whether it be chlorinated solvent or aqueous.
Emulsions are invariably formed in the hot state since the surface tension of water at 90°C is 61 dynes per square centimeter as against 73 dynes at 20°C; also the viscosity of both oil and water is a linear function of temperature, water losing 70 per cent of its room temperature viscosity at 90°C, i.e., one poise at 20° reducing to 0.31 poise at 90° and animal oil losing 90 per cent of its room temperature viscosity at 100°, i.e., 0.42 poise at 30° reducing to 0.046 poise at 90°. The above reductions in viscosity-and surface tension account for the ease with which emulsions are made at elevated temperatures.

Emulsions are either water-in-oil (w/o) or oil-in-water (o/w), depending primarily on the selection of an emulsifier. Fatty soaps containing monovalent cations yield o/w emulsions. Fatty soaps containing bi and trivalent cations yield w/o emulsions. Emulsifiers which are strong in hydrophilic elements (polyoxethylene) yield o/w emulsions and conversely emulsifiers strong in hydrophobic elements (mono and di esters) yield w/o emulsions. Water content is not critical regardless of which type emulsion is desired, namely, o/w or w/o, but if the water content is not within reasonable limits the continuous phase can become discontinuous; in other words, the emulsion may invert from o/w to w/o in emulsions containing a deficient amount of water in the order of 10 to 20 per cent instead of the nominal amount of 50 to 60 per cent. The determination of which phase is continuous is not difficult. The most common method being to determine whether water acts as a diluent; if so, obviously water is the continuous phase. However, some emulsifiers are sufficiently vigorous so that either of the following two methods must be used: (1) the addition of an oil or water soluble dye and (2) the conductance of an electric current showing water to be continuous.

In 1820 Brown, an English botanist, first discovered that matter in a finely divided state when suspended in water was in violent agitation, which has since been termed Brownian movement. In 1879 the great physicist Helmholtz theorized that each particle in Brownian movement must be surrounded by a layer of adjacent anions which were again enveloped by an equal number of cations. The potential across this rigid double layer as hypothesized by Helmholtz was denoted as sigma, S, which is the transverse potential of the rigid double layer. In 1910 Guoy and Freundlich theorized that a portion of the cations as described by Helmholtz must be in the mobile liquid and ascribed a potential to these mobile cations only as a ”zeta (Z) potential” and referred to them as a tangentical potential. The character of the transverse potential curve S, of which the tangentical potential curve Z is part, is dependent on the concentration of the electrolyte and valence of the ions; as these latter two increase, the potential and the double layer thickness decrease until precipitation occurs.

From the foregoing it appears that suspensoids are much more difficult to process than are emulsoids and fortunately protective colloids can be employed which envelope the suspensoids and their double layers, causing the suspensoids to behave exactly as an emulsoid thereby tolerating the electrolytes formed by the hydrolysis of the salts and hydrates present in the abrasives (the combined RO and R203 in Rose tripoli powder averages 4.6 per cent and the white cryptocrystaline silicas 0.25 per cent).

Zsigmondy determined the relative strength of certain negative protective colloids and defined his ”Gold Number” as the number of milligrams of dry material of the emulsoid sol (protective colloid) which is just sufficient to prevent the change from red to blue in 10cc of gold sol, after the addition of 1cc of a 10 per cent sodium chloride solution. The reciprocal of the Gold Number is the ”U Number” which readily conveys the comparative effectiveness of the colloids.

An interesting and significant research project² employing the turbidimetric method to determine the approximate particle size average in individual emulsions shows very close agreement with the average particle size as determined by means of the electron microscope.³ The turbidimetric method utilizes a Parr Instrument Type S-3 visual turbidimeter which measures the ”extinction path length” or depth of the solution through which a light source just fails to be visible. By means of the electron microscope the authors⁴ determined the particle size by extrapolating data pertaining to an emulsion of Carnauba wax, borax, and fatty soap (excluding waters for dilution and to make the emulsion), 0.0765 microns for a 10.3 per cent solution of soap and 0.0685 microns for a 11.9 solution of soap. It appears that the minimum, and also the optimum, particle size for a fatty soap emulsion of Carnauba wax lies between 0.06 and 0.075 microns.

Bolton and Marshall, by means of a visual turbidimeter, made the following determination from emulsions of an oxidized paranic wax, oleic acid, and morpholine; the particle size increased with decreasing oleic acid additions over a wide range of morpholine additions, the particle size decreased as the oleic acid increased even though the morpholine additions were at a lesser percentage. From these data it is concluded that the sensitivity of a system to added morpholine is decreased with decreasing oleic acid and that relatively more morpholine is required for emulsification as the oleic acid is decreased, and also that the particle size is decreased greatly as the oleic acid increases and morpholine decreases.

Electron micro photographs of an emulsion by Schoenholtz and Kimball show the effect on the dispersed phase of using an excessive, a deficient, and the optimum amount of an amine emulsifier to gain minimum particle size. An excessive amount of amine causes the formation of agglomerates of such a large, irregular and random shape that one quarter of the visible field is heavily shadowed. A deficient amount of amine yields an entirely different type dispersion which consists partly of the spherical emulsoidal particles but contains mostly an unusual amount of anisotropic or crystalline lattices appearing as twisted ribbons and ;n some cases like finely dispersed fibrils none of which have similar configurations. The optimum amount of amine causes a dispersion- which is associated with the normal concept in that the dispersed particles are,- in some cases, truly spherical and some slightly elliptical, some spheres appear to have nodes attached but this may be due to one sphere partially underlying another and appearing as an appendage due to the dimensional-aspect of a photograph. It is significant to relate here that the normal concept of metathetical reactions as between oleic acid and an amine do not hold true in dealing with colloids as is shown above, and which confirm further the literature,⁵ which states that simple stoichiometric relations no longer exist between the added substances.

It is of paramount importance that the components in the oil phase and the emulsifiers used in making the emulsions in which- abrasives are suspended, produce soils which are adapted to the method of cleaning; for example, soluble fatty soaps are excellent emulsifiers but are insoluble in organic degreasing solvents; such soaps are readily removed by aqueous soak, spray wash, and electro cleaners. Surface active agents are very excellent emulsifiers, certain of them are cleanable in every cleaning method used prior to electro deposition; and are unique in that certain of them solubilize oils, fats, and waxes with the- exception of sterols. These highly complex alicyclic alcohols and their esters are among the finest wax type lubricants known but also one of the most difficult to clean when in the form of buffing soil. Certain of the more common phosphatides are used as emulsifiers and also assist the more vigorous emulsifiers, however, they hydrolyze to form quaternary bases,⁶ which are cationic and can cause cleaning difficulty because of the incompatibility of anionic and cationic surface active agents. The cleanability of liquid bung composition soil is dependent, quite naturally on its components but the quantity of soil is equally important; large amounts of easy cleaning soil are apt to leave patterns under plate if the buffed work is excessively loaded with soil. Such heavy soil removal is not a linear function of cleaning time and operators should beware of extreme soil loading. Solid or bar type compositions for nonferrous metals contain approximately 30 per cent binoer, while liquid compositions contain approximately 40 per cent binder. This 10 per cent increase, which is further accentuated by the accompanying 10 per cent drop in abrasive powder, would-seemingly cause liquid compositions to transfer more soil to the buffed work; however, the evaporation of the water from the buffing head cools the fabric very noticeably causing the grease face to become firmer and/or the fabric to become more rigid and therefore wipe the soiled work to a higher lustre than in the case with bar compositions; also, the water wets the cotton fibrils which have a higher tensile strength wet than dry.⁷ Cotton is unique among the linear polymers in that it is one of the very few exhibiting this property.

Trichlorethylene degreasing solvent has been replaced by perchlorethylene where more stubborn soils require greater heat and greater solvency for their removal.⁸ Mr. J. Hensley⁹ and coworkers have used carbon C-14 tagged stearic acid as a soil applied onto highly polished steel discs finished with No. 220 silicon carbide in order to determine the effect of cleaning time with various singly used alkalis. In the course of their work it was found that 0.8 microgram of stearic acid on steel would wet perfectly and take a satisfactory bright nickel plate. This amount of soil, if assumed to be evenly distributed, and assumed that the surface were perfectly smooth and the effective and macro surfaces were equal, would be four molecular layers thick or 1.0 millimicrons. However, it cannot be stated arbitrarily that a certain quantity of soil will have a specified effect on the wettability of the metal surface or the nature of the electroplate applied over it.

The density of the silicas and the alumina which are suspended in the emulsions are two to five times greater than the emulsion density, and for the suspension to be free of settling obviously the emulsion must be stable and non-creaming (showing no sign of coalesced dispersed phase floating on the surface). The emulsion particle size should be reduced as far as possible (approximately 0.06 micron diameter) to attain this stability and protective colloids such as natural or synthetic gums can be used to raise the viscosity of the continuous phase to prevent settling.

Liquids are classified with reference to viscosity as Newtonian, Dilatant, or Thixotropic depending on whether they maintain viscosity independent of the speed- of agitation, such as water or oil (Newtonian); whether they become more viscous on agitation, such as egg albumin or whipping cream (Dilatant); or whether they become less viscous on agitation such as Bentonite clay slurries and sols of iron oxide and aluminum oxide (Thixotropic). Liquid buffing compositions fall into this latter class, the name being derived from the Greek, thixis, meaning ”touch,” and tropein, meaning ”to change.” ”Plastic Set” and Thixotropy are synonymous terms and refer to the ”livered” or gelled condition of the liquid suspension. This pseudo gel is short lived, however, when the liquid is being pumped or in any way agitated and will develop again on standing. Because of the tremendous reduction in viscosity due to agitation inherent in pumping (a reduction from 12,000 cps to 6,000 cps is not uncommon) it is well to reduce proportionally the regulated pressure at the pump after a short period of recycle. The phenomenon called Thixotropy, which is beneficial when controlled in liquid buffing- compositions since it prohibits the abrasive from; settling on storage, is caused primarily by the solubilized salts and hydrates present in the abrasives. Thixotropy increases proportionally to the content of high melting components present in the dispersed phase of the emulsion when solvents are excluded; it also is a direct function of the alumina content of siliceous abrasives (Rose tripoli powder contains roughly four times the alumina as does Cream tripoli, while the more pure silicas contain half as much alumina as Cream tripoli).

The rate of metal removal by buffing or ”cut” has been a relative term having no coefficient values except in the laboratory where pre weighed brass discs work against each other in the presence of abrasive and mineral oil. A more significant laboratory testing device has a moving bed under a pressurized buffing head; however, in practice the criterion for cutting power rests with the operators and inspectors. The most satisfactory means for determining ”cut” in the field is intentionally to coarse polish the base metal and follow with three or four passes of the buffing head at uniform pressure and equal elapsed time; the amount of coarse polish showing through the finish has proved quite satisfactory in determining comparative ”cut.” The cutting power of a liquid composition is very considerably dependent on how well the liquid ”faces” the buffing head; almost invariably a faster cut is produced by a firm, hard wheel face for it is self evident that the abrasive must be firmly held by the wheel in order to cut. The success of liquid compositions in industrial application is due mainly to the attainment of a firm wheel face which is nearly synonymous with cutting power.

An ideal emulsion would be one that would split phase the instant it becomes atomized, to allow for free evaporation of water and cause the dispersed phase to coalesce onto the continuous grease face on the bung head and also to prevent re-emulsification once the binder has split from the emulsion. Fortunately high vapor pressure emulsifiers are available which evaporate with the water and prevent re-emuIsification on the wheel face. The water content of the emulsion should also therefore be just in excess of the critical for phase inversion so that even slight evaporation causes the dispersed phase to coalesce. The mono, di, and tri valent metallic oxides present in siliceous abrasives unfortunately cause gel formations which require additional water for dilution. The more pure hard-buffing abrasives now being marketed are sufficiently low in the above metallic oxides so as to allow the water content of the emulsion to approach the critical inversion point and thereby reduce the wheel throw off.

Before concluding it should be stated that the design and performance of the spray equipment has caused both failure and success of many liquid composition formulations; a change of spray gun design in the field has caused mediocre formulations to perform quite satisfactorily.

It is hoped that the information correlated in this discussion serves to enlighten those concerned and thereby add to the progress of metal finishing and electroplating.

REFERENCES
1. L. G. Lundsted, U. S. Patent No. 2,674,619, assigned to Wyandotte Chemical Corp., April 6, 1954.
2. M. E. Bolton and A. W. Marshall, ”Examination of Wax Suspensions by Turbidimetric Methods,” Soap and Sanitary Chemicals (Sept., 1949).
3. D. Schoenholtz and C. S. Kimball, Soap and Sanitary Chemicals (Aug., 194?).
4. Ibid.
5. Kruyt and Van Klooster, 2nd ed., p. 5 (1930).
6. J. B. Conant, The Chemistry of Organic Compounds, p. 186
(1933).
7. S. Setterquist, National Lock Company, Rockford, III.
8. C. W. Smith, Whitfield Chemical Co., Detroit, Mich.
9. J. W. Hensley, H. A. Skinner, H. R. Sutter, ”A Metal Cleaning Test Using Radioactive Steric Acid as Soil,” Special Technical Publications No. 115, American Society for Testing Materials (1952).

DISCUSSION

MR. LAVERNE VERZIER (Consultant, Waterbury, Conn.): Have you found too much effect from the impurities in the stainless steel cutting grades or stainless coloring grades on the stability of your emulsions?

MR. TWYNING: No, if the protective colloids are used to control viscosity of the continuous phase then the electrolyte formed by the soluble impurities does not cause precipitation of the suspensoidal particles since these particles then assume the same properties as the emulsoidal particles, but if the protective colloids are not used then the gravity of the abrasive being so much greater than that of the emulsion you get settling. However, in some cases we have had gas evolution from liquid stainless composition which is hard to explain.

MR. EDWARD HERZIG (Electro Tec Corporation, South Hackensack, N. J.): Can colloids be used to improve finish over regular type polishing compounds? If so, which of the liquid buffing types?

MR. TWYNING: Normally, in finishing nonferrous metals, the finish color of the metal is higher than when bar composition is used; this is so even though the liquid composition may contain 10 per cent more binder than bar type. It is probable that the water from the liquid type keeps the buff face cool and therefore firm.

MR. HERZIG: Are the particles of colloidal size?

MR. TWYNING: Of the abrasive?

MR. HERZIG: Yes.

MR. TWYNING: Oh, yes, they run from one-tenth to three-tenths micron diameter.

MR. HERZIG: So actually you cannot get a better finish.

MR. TWYNING: You can use the same abrasive in liquid composition as in bar and the finish will not be any different.

MR. HERZIG: What would be the proper type of equipment you have mentioned—is it bung wheels or type of wheels?

MR. TWYNING: No, it is not dependent on the manufacture of the wheel or type of wheel. Normally, the bias construction buff is used on automatics where liquid compositions play the greatest part. It does not have to be all biased buff, many times piece buffs and bias bus are put together 2-to-1 or 1-to-1.

MR. HERZIG: But what is the proper type of equipment which you have mentioned?

MR. TWYNING: Oh, pardon me I see what you meant. The construction of spray equipment is quite technical and you almost have to become involved in this field to appreciate how technical it is; making spray equipment is involved and is quite an art and unless the equipment is proper and of rugged construction so that the vibrations inherent on an automatic machine do not tend to loosen the nuts and packings, you can have trouble. Spray guns should be rugged so that rough handling in use doesn’t prevent proper performance. The airports in the wings of the air cap allow for pattern control which is vital cost wise since overspray is wasteful. Proper gun design and performance are necessary in order to place all of the composition on the wheel face.

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New transistor analyzer, small, lightweight, and portable between laboratories, was developed by Armour Research Foundation engineers. The instrument is used to measure directly the circuit constants of transistors while they are in operation.

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Ceramic coating of extremely low thermal neutron absorption coefficient was developed by National Bureau of Standards for high temperature protection of alloys in nuclear reactors. Coatings were said to withstand thermal shock resistance tests involving quenching in water from as high as 2,000° F.

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Battery operated survey meter for neutron measurement was developed for protection of personnel around atomic accelerators and reactors by Nuclear Instrument and Chemical Corporation, Chicago. Unit contains two neutron detectors, one for thermal neutrons, the other—surrounded by a block of paraffin and a cadmium shield—for ”fast” neutrons with energies of more than 1 ev.

New surface anesthetic not only provides prompt relief from surface pain but also low toxicity and low sensitization, according to Abbott Laboratories, scientists who developed it. New drug, called tronothane, is said to afford unusual freedom from skin irritations often encountered with topically applied anesthetics.

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A new welding electrode is said to provide better weld quality in inert-gas, shielded arc welding of aluminum, according to Aluminum Company of America. The new electrode also can be used for tungsten-arc welding where filler wire is fed mechanically to the work.

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A simple method for producing furfuryl alcohol resin, invented by Erik R. Nielsen, senior chemist at Armour Research Foundation, provides forbetter and cheaper manufacture of plastic pipe, hot-melt coatings, and molding powders. Resin intermediate is formed quickly and smoothly by heating furfuryl alcohol in the presence of an activated alumina—without use of strong acid catalyst as was required formerly.